1. Field of the Invention
The invention relates to a safety circuit for a servo-hydraulic regulating system for the hydraulic control and actuation of a load unit, in particular of operating tools on utility vehicles and agricultural machines.
2. Background of the Invention
Servo-hydraulic regulating systems are needed, in particular, in mobile hydraulics, where it is important to provide the user of an operating tool with a maximum of operating or accident safety. More particularly, vibrations arising from bumps during the usage of utility vehicles and agricultural machinery can be transmitted to their operating tools. Because of their large mass, typical operating tools can thus exert a considerable influence upon the pitching vibrations of the vehicle. Accordingly, servo-hydraulic regulating systems are required which essentially serve as a guide mechanism of the operating tool for maintaining a pre-set position relative to the vehicle and as a facility for compensating for lifting and pitching vibrations of the vehicle.
The safety requirements placed upon machinery and tools of the above-described type have recently become increasingly strict. In the field of servo-hydraulic regulating systems, such as servo-hydraulic lifting gear regulating systems on tractors (SHL-systems), the safety functions are inferior, in part-areas, to those of electro-hydraulic systems. Accordingly, the absence of certain safety functions is a distinguishing feature between servo-hydraulic and electro-hydraulic systems.
Servo-hydraulic lifting gear regulating systems, such as those shown in German Offenlegungsschrift 2940403 or German Offenlegungsschrift 3106086, operate with a main valve, in the form of a directional control valve or proportional valve, which is pressure-actuated in two directions and which is controlled by a control circuit having a hydraulic measuring bridge with fixed and variable restrictors. One of the variable restrictors serves as a set-value generator and the other as a regulating variable sensor which is mechanically coupled to the moved part of the lifting gear. When, via a variable restrictor, a new set value for the lifting gear has been set, the directional control or proportional valve is displaced from the neutral setting and only reverts to there once the hydraulic measuring bridge is balanced, i.e., once the control pressures bearing against the proportional valve piston are equalized.
In previous servo-hydraulic lifting gear regulating systems, if the regulating system is switched on, i.e., the motor is started, when the measuring bridge is out of tune, a sudden and unexpected regulating reaction will give rise to a dangerous lifting gear movement. In that regard, the measuring bridge can become unbalanced while the motor is idle as a result of one or more measuring orifices falling out of tune, i.e., for example, as a result of lever shift, change of force at the force sensor, or change of location of the lifting gear.